directions:
1. Make 1 cup of Parmigiano-Reggiano slivers with a vegetable peeler. Set aside.

2. Slice the potatoes very thinly, sweet potatoes first, then all-purpose. As soon as potatoes are sliced, melt butter and olive oil together in a small pan. Pout half into a 9" cast iron skillet set over low heat.

3. Make a layer using 1/3 of the white potato slices, overlapping them in the bottom of the skillet, cycling in a spiral from the outside edge to the center. Sprinkle lightly with salt, pepper and nutmeg. Scatter 1/5 of th Parmigiano-Reggiano slivers over the layer.

4. Next make a layer using 1/3 of the sweet potatoes, overlapping them in a spiral going in the opposite direction. Season and scatter with Parmigiano-Regginao slivers. Continue making layers of potato slices, seasonings and cheese. The 6th and final layer will be of sweet potatoes. Seasons and pout remaining melted butter and oil over the top.

5. Cover pan tightly with foil, crimping edges to seal. Bake in a pre-heated 425-degree oven for 25 minutes. Remove foil and continue cooking 25 minutes, until potatoes are tender and browned at the edges. Cut into wedges and serve immediately.